Discussions

Current limiting works most of the time by sensing the voltage drop across a really small value resistor in series with the load (in this case the battery.) and using that to turn off a transistor also in series as the voltage across that resistor becomes too high. This has the effect of current limiting.

There are several transistor circuits online which show how this could be done. My favorite is the classic open collector current mirror because at first glance it makes no sense! Learn about all the easy ways to do it here:

I can't tell if you have a battery with 2 cells or 3 cells in series, (as the nominal voltages would be either 7.4v or 11.1v respectively.) Regardless, once the cell reaches 4.20v per cell, then the voltage should be limited to exactly 4.20v. That is easy with adjustable regulator or emitter follower set to a little over 4.2v to feed the current sink or source. Just note that the current sink or source is trying to maintain a fixed voltage drop across the shunt resistor, and there is also a diode drop as part of transistor action. This can be as much as a few volts.

The hard part would be to make it so the voltage of the individual cells are individually regulated. It is important because otherwise what could happen is that small differences in capacity or internal resistance between the cells causes the voltage on one cell to be higher than the other, pontenially damaging the overcharged cell.

A Li on battery will fully charge to a maximum of 4.2 volts per cell so 2 cells are 8.3 v and 3 cells 12.7 v

They will quickly discharge under load to about 4 volts or so.

You can't have a 9 volt LiON or LiPO .

It is critical that you understand what charging Lithium batteries is about. If you over charge them OR allow them to become under charged then they may. can , will burst spontaneously into flames. NOT nice in the house car or garage.

The links I gave should have given you some theory and background enough that a competent electrical handyman should be able to build a charger.

Over discharging them leads to failure on recharging. I run electric RC aircraft and twice I have inadvertently allowed the battery to discharge below 2 volts per cell. Charging one was OK but the other puffed up and blew all the magic smoke out :-( Luckily I charge outside in a flame proof container.